The plates are 2.5mm thick.

I had a couple of ideas for a holder. Cannibalize one of my 4x5 film holders and insert a piece of 4 x 5 matt board with a 2 1/2" square opening. Or try and modify an old 4x5 dry plate holder I have. That's about as far as I've gotten.

Yes, a third person would be even better. If I can't get something in 10 tries, then I'll probably throw in the towel.